1. Field of the Invention
The invention relates to a method for evaluating the signal of an optoelectronic position and angular position measuring device with an encoder disk that can move relative to transceiver having a light source and a sensor array provided with a number of optoelectronic transducers. The invention also relates to a preferred use of such a process.
2. Background Art
Position and angular position measuring devices are frequently used for automatic positioning and measurement in machine tools and in coordinate measuring instruments. For example, rotation angle sensors are used in the area of motor vehicles to determine the absolute angular position of the steering wheel, and thus they are also called steering angle sensors. Such optoelectronic steering angle sensors essentially include a rotor and a stator. The rotor is an encoder disk coupled to the rotational motion of the steering wheel and the stator is a transceiver. The encoder disk includes a light source, for example an LED system, located on one side of the encoder disk. A receiving device, such as a line sensor having numerous adjacent transducers, is located on the other side of the encoder disk. From time to time, the coding used by the encoder disk is a digital code which includes several parallel code tracks and is built, for example, in the manner of a Gray code.
A value for the steering angle is needed in motor vehicles, for example, for use by a vehicle movement dynamics control system. In addition to the mentioned steering angle values, such a vehicle movement dynamics control system receives other measurement data, such as the wheel speed or the rotation of the motor vehicle about its vertical axis. The absolute steering angle and the steering speed are needed so that these values, along with the other captured data, can be evaluated by the vehicle movement dynamics control system and converted to control actuators such as the brakes and/or the engine management system.
Previously known position and angular position measuring devices evaluate the signal amplitudes of the transducers assigned to each code track by comparing the signal amplitude with a specified signal threshold. If the signal threshold is exceeded, the signal amplitude of the transducers assigned to this code track is evaluated as exposed by the coding of the code track. If the signal amplitude is smaller than the specified signal threshold, an evaluation is made that the coding of this code track is not causing exposure of the transducers assigned to this code track.
Assignment of certain transducers to a code track is used in the previously known process to compensate play of the encoder disk in its movement relative to the transceiver. For this purpose, the coding has at least one reference track assigned to it. Because the distance from the code tracks to the reference track is the same over the length of the code tracks, it can be determined which code track is exposing which transducers by determining the position of the reference track on the sensor array.
Although the previously known process can achieve sufficiently precise results when the conditions which have an influence on signal detection remain constant, such position and angular position measuring devices can provide incorrect measurement results if, for example, environmental influences should change the idealized measurement parameters specified during the design of the position or angular position measuring device. Especially when such an angular position measuring device is used as a steering angle sensor in a motor vehicle, exact angle detection should be ensured under the most diverse influences.
For example, such a measuring system can be affected by stray light, or by the encoder disk being covered with condensation or ice or even dust, and the signal threshold is often exceeded in the examples mentioned not only when there is an actual direct exposure by the coding of a code track, but rather also due to stray light exposing the transducer elements assigned to such a code track. In a corresponding manner, it is also possible for the signal threshold not to be exceeded despite being exposed by the coding of a code track, when there is dust, for example. In these cases the angle value read from a steering angle sensor is faulty; accordingly, the subsequent evaluations and analyses based on this value are also faulty.
Therefore, starting from this prior art that has been discussed, the invention is based on the task of further developing a process of the type mentioned at the beginning in such a way that it is able to avoid the disadvantages shown above, at least as much as possible.
This task is solved according to the invention by the fact that the method contains steps which make it possible for the evaluation and/or analysis of at least one measurement parameter of the signal amplitude produced when a transducer of the sensor array is exposed to be adapted to changing measurement conditions. The steps of the method include:
capturing the amplitude of a reference signal;
evaluating the amplitude of the reference signal; and
adapting at least one of the parameters that is relevant for evaluating the amplitude.